The specific aims of this study are to evaluate myocardial thallium-201 kinetics in both an intact and isolated experimental model and also to determine the specific effects of hyperemia. Studies will be performed in hearts that are either normal, ischemic, hypoxic, metabolically blocked or exposed to glycoside infusions. An acute dog preparation having a left circumflex artery infusion of adenosine will be used to study the selective increase in coronary flow on regional thallium-201 kinetics. These hearts will be analyzed to determine the initial uptake and redistribution of thallium-201 in an intact heart. An isolated rabbit heart having a constant flow rate (high, low and normal) will be used to study the uptake and washout of the tracer during different cardiac states. These hearts will yield information concerning both the cellular effect of digitalis, hypoxia and altered metabolites on myocardial thallium-201 kinetics, as well as overall flow changes. The basis for the clinical use of thallium-201 myocardial perfusion scans to detect coronary artery disease relies on a disparity in regional coronary perfusion during the tracer administration. The disparity in coronary flow can be achieved by exercise or pharmacologic vasodilation, and subsequent cardiac images can demonstrate myocardial regions that are normal, ischemic or infarcted. However, thallium-201 myocardial uptake and release have not been critically evaluated during maximal vasodilation, and controversy exists even concerning normal kinetics. The potential significance is that a better understanding of thallium-201 kinetics at normal and hyperemic flows will permit better interpretations of clinical studies.